In EP 2 091 067 A1, an apparatus for treating a substrate in a vacuum chamber is disclosed. In this vacuum chamber n cathodes and n+1 anodes are arranged, each of said anodes adjacent to a cathode. Each of the n anodes and n of the assigned cathodes are connected to a power supply. One of the anodes is, not assigned to a cathode, connected to an electric line connecting each of the anodes. A pull down resistor is connected to said line at its one end and to ground at its other end.